The present invention relates to a central differential for a four-wheel drive motor vehicle, and more particularly to the structure of the central differential.
In a four-wheel drive motor vehicle, a theoretical torque split of the front wheels and rear wheels of the vehicle is determined in accordance with a dynamic weight distribution of the front and rear wheels, considering the movement of the center of gravity of the vehicle at acceleration so that the power of the engine may be used most effectively.
In the four-wheel drive vehicle with a front-mounted engine, which was developed from a front-wheel front-drive type, the ratio of front torque T.sub.F to the front wheels and rear torque T.sub.R to the rear wheels are in the ratio of 50:50. In the four-wheel drive vehicle with a front-mounted engine, which was developed from a front-wheel rear-drive type, the front torque T.sub.F and the rear torque T.sub.R are in the ratio of 40:60. The central differential comprising bevel gears is employed in the former system in which the output torque of the engine is equally distributed, and the central differential having a simple planetary gear device is employed in the latter system.
The former system ensures safe driving on a slippery road. If a differential lock device is provided for locking the central differential, the driveability of the vehicle is improved further. However, when the vehicle makes a turn at high speed under the differential lock condition, all of the four wheels may slip (slipping spin) at the same time, causing difficulty in driving.
In order to ensure driving stability of the vehicle, the torque to the rear wheels is set to a value larger than that to the front wheels by arranging the central differential comprising the simple planetary gear device, so that the rear wheels may slip first. Thus, cornering performance is improved by the front wheels at a small torque while the rear wheels idle.
Japanese Patent Application Laid-Open 63-176728 discloses a four-wheel drive motor vehicle in which a central differential comprising a simple planetary gear device is provided. The output of a transmission is transmitted to a carrier of the planetary gear device. The torque is distributed to the front wheels through either a sun gear or a ring gear and to the rear wheels through the other gear. The torque to the front and rear wheels is unequally distributed at a ratio determined by the difference between pitch circles of a sun gear and a ring gear. A fluid operated multiple-disk friction clutch as a lock device is provided for controlling the differential operation.
U.S. Pat. 4,523,495 discloses a central differential comprising a double-pinion planetary gear device for the four-wheel drive motor vehicle. In the system, a final reduction gear of a transmission is used for a carrier of the differential. The differential has three pairs of double pinions. Each pinion is rotatably mounted on a shaft secured to the carrier. One of each pair of pinions is engaged with a sun gear formed on a differential case of an axle differential for a front-wheel, and the other pinion is engaged with a sun gear for transmitting the power to the rear wheels.
Meanwhile, in the four wheel drive vehicle developed from a rear-wheel drive vehicle with a front-mounted engine and in the four wheel drive vehicle developed from a front-wheel drive vehicle with a front-mounted engine, the central differential is mounted in the rear of the transmission. Consequently, the space in the vehicle is reduced by the central differential, which affects room (space) and design for mounting various equipment. Therefore, it is necessary to reduce the size of the planetary gear device of the central differential. Generally, in order to reduce the size, the diameter of the pinion is reduced. In such a case, a plurality of pinions, for example three and four, are provided for dispersing the load exerted on the teeth surfaces of the pinions.
When the planetary gear device having a plurality of pinions is used, conditions of engagement of the pinions and the number of teeth of each pinion should be determined, so that the pinions may be disposed at equiangular disposition. In the simple planetary gear device, if the number of the pinions to be used is n, the number of teeth of a ring gear is Z.sub.R, and the number of teeth of a sun gear is Zs, and for evenly spaced pinions the equation is determined as follows. EQU m=(Z.sub.R +Zs)/n
(m is an arbitrary integer)
In the double-pinion planetary gear device, if the number of the double pinions to be used is n, and the number of teeth of two pinions are Z.sub.s1 and Z.sub.s2, respectively, and for evenly spaced pinions the equation is determined as EQU m=(Z.sub.s1 +Z.sub.s2)/n
(m is the arbitrary integer)
Further, in the double-pinion planetary gear device system, the pinions of each pair are disposed adjacent each other and engaged with each other. Consequently, it is necessary for have the basic conditions of the pinions such as module, pressure angle and helix angle to coincide with each other. Accordingly, the design variation is limited by the basic conditions.
In the system having the simple planetary gear device, the theoretical torque split determined by the ratio of the pitch circles can not be changed, unless the diameters of the sun gear and the ring gear change. In the double-pinion planetary gear device, the theoretical torque split can not be changed, unless the diameters of the sun gears change. However, it is difficult to change the diameters because of the limitation of space. Thus, the range of the theoretical torque split can not be widely expanded.
If the number of the teeth is determined in accordance with the theoretical torque split regardless of the above described engagement conditions, pinion couples are disposed at irregular angular spaces.
In the system of the double-pinion planetary gear device, the central differential is coupled with the final gear, thereby reducing the size. However, a differential lock device must be provided separately from the central differential, so that the whole system becomes large in size. Since the final gear is cantilevered to support the central differential, the final gear is subject to be tilted. This will generate noise, and a reduced durability of the gears and bearings.